This disclosure generally relates to systems and methods for tracking the holonomic motion of an automated tool (such as a non-destructive inspection unit) as it moves over a target area.
Portable non-destructive inspection (NDI) instruments that utilize hand-held sensors are common for conducting manufacturing and in-service inspection of selected areas of composite structures. Most in-service inspection of aircraft structure is done using hand-held sensors. Small parts or areas are occasionally inspected this way during manufacturing as well.
While hand-held sensors are commonly used because they are lower cost, as compared to automated scanning systems, an expert is needed to interpret the resulting signal. In addition, searching for damage or defects by hand while watching for a signal change is time-consuming and is prone to operator error or fatigue. Quantifying the damage takes time as well, since a point-by-point measurement is being made. It is also well known that many hand-held NDI sensor units do not detect smaller damage/flaws that could be detected using scanning that can create images of the area containing the damage. This image-based scanning relies upon correlation of the sensor positional information relative to the sensor data itself. Correlation of single-point NDI data acquired by a hand-held sensor with position is difficult. Also, higher damage detectability limits of common hand-held sensor units mean the inspection cycles are shorter (and costlier) than they could be.
Currently available solutions for tracking of hand-held scanning devices usually track only a single axis in the direction of travel (i.e., one-dimensional motion). This one-dimensional tracking is typically performed using a wheeled rotational encoder. But with free-form movement of the operator's hand providing the motion for the scanning device, there are no guides to ensure that the sensor (e.g., a linear transducer array) is traveling in a straight line. The result is that the one-dimensional tracking of the wheeled encoder may provide very poor tracking of the actual motion of the operator's hand. A skilled operator may be able to partially compensate for this by concentrating on making straight line motions, but humans are not very good at making straight line motions, especially over long distances. Wavy scans are often the result—although this waviness is not apparent to the operator during the scanning process, since the output from the one-dimensional scan will be misleading by appearing to be straight.
Real-time imaging of hand-scan NDI data is currently performed using several different types of position tracking methods, including: single axis rotational encoders, X-Y bridges, scan arms, trackballs, and ultrasonic or optical triangulation are some of the most common. The biggest problems with hand scanning of a single probe are: data positioning (shifts occur depending on the direction of probe movement or where the probe stops) and getting enough passes to cover an area. Drift is also a problem. The two-dimensional images acquired using free-hand motion are usually not very accurate representations of the actual path taken. A linear array, for some methods like ultrasonic and eddy current, will improve the image by making bands of data that accurately represent the relative positions of the data within each band. The biggest problem with hand scanning using linear arrays, however, is maintaining or determining orientation (if not constrained).
During NDI surface scanning applications using hand-held devices, the operator is usually required to maintain a consistent orientation of the scanning device with respect to the direction of travel in order to capture an accurate representation of the scan region. In addition, the operator needs to maintain the velocity below a maximum speed in order to avoid missing data. These requirements can be difficult to accomplish without a measurement system capable of providing feedback to the operator.
These inaccuracies in position measurement, along with the lack of orientation and velocity feedback, have a negative impact on performance. Operators of hand-held scanners may compensate for the inaccuracies in translation and rotation, and the lack of velocity feedback by moving more slowly, which can lead to longer amounts of time needed to acquire useful scan imaging data. This can sometimes lead to re-work to address areas missed during the scan.
There is a need for methods that determine sensor position and orientation in order to obtain correctly aligned NDI images from data acquired during free-form motion.